The invention relates to a method for producing a carbon band twisted about a longitudinal axis for a carbon infrared heater.
Furthermore, the invention relates to a method for producing a carbon infrared heater, comprising the preparation of an envelope tube made of quartz glass, into which is inserted a carbon band twisted about its longitudinal axis, and whose ends are provided with electrical terminals, which are led out from the envelope tube.
In addition, the invention involves a carbon infrared emitter having an envelope tube made of quartz glass, in which is arranged a carbon band containing carbon fibers, which is twisted about its longitudinal axis and whose ends are provided with electrical terminals, which are led out from the envelope tube.
Infrared emitters having a heating element made of carbon fibers distinguish themselves by a high reaction rate and thus allow particularly quick temperature changes. From German published patent application DE 198 39 457 A1 a method is known for producing a carbon band wound in a spiral shape for an infrared emitter. To this end, a band-shaped starting material is used, in which carbon fibers are embedded in a thermoplastic embedding compound. After heating the starting material to the softening temperature, the embedding compound softens, so that the band-shaped material can be wound in a spiral shape onto a mandrel. Through carbonization the embedding compound is converted into a carbon and in this way the spiral-shaped carbon band is fixed in its shape, so that later plastic deformation is prevented in its proper use as a heating element in an infrared emitter.
The known method allows the production of spiral-shaped heating elements from a carbon band. Due to the spiral shape the surface of the resulting heating element is significantly larger than the surface of a cylindrical, elongated heating element of equal length and thus generates a higher radiation power output (at the same temperature).
In European patent application publication EP 1 619 931 A1 an infrared emitter is described in which the heating element is present in the form of a twisted carbon filament. With respect to the production of the twisted filament, it is disclosed that it is generated by pressing a plurality of carbon films, which are layered one above the other and are connected rigidly to each other. It is furthermore to be inferred that, for producing the electrical terminals at the two ends of the filament, thin metal networks are provided that would be embedded between layers of the carbon films during the pressing process. A secure connection should allegedly result between the electrical terminals and the carbon filament.
It is not explained how a solid body in the form of an elongated band or a mechanical joint connection between carbon films and a metal network should be able to be produced through simple pressing of layers made of carbon films. In addition, there is the risk that the embedded metal network experiences carbonization, due to heating during the operation of the emitter, through the contact with the carbon from the carbon filament. This carbonization can lead to changes in the crystal lattice and to carbide formation in the metal, so that hardness, strength, and coefficient of thermal expansion change and, in particular, the electrical conductivity becomes worse. This worsening of the electrical conductivity generates additional heating during operation and thus an accelerated conversion into carbide.
A suitable way of contacting carbon bands for heating emitters is known from European Patent EP 0 881 858 B1. Therein, the production of carbon infrared heaters having filaments from elongated bands is described, which is obtained by heating processes from unidirectional, fiber-reinforced thermoplastic. For contacting, bonded thick sections are provided on the band ends, which are fixed and held by springs made of molybdenum sheeting.